The present invention relates to an optical transmission system which utilizes an optical fiber or space as an optical transmission medium and, more particularly, to an optical amplification system which amplifies signal light in any state of polarization.
Practical optical fiber communication systems currently utilize an opto-electronic and electro-optical conversion for repeating optical signals, which involves converting an optical signal into an electric signal, amplifying and waveformshaping the electric signal, and driving a semiconductor laser by the amplified electric signal.
On the other hand, there has been proposed, as a new way of repeating optical signals, an optical amplification system, which directly amplifies an optical signal. A repeating system utilizing optical amplification is advantageous over conventional opto-electric and electro-optical conversion repeating systems, since circuits in a repeater can be simplified in optical amplification systems and since wavelength-division-multiplexed signals or frequency-division-multiplexed signals at a desired transmission rate can be also amplified simultaneously by one optical amplifier. Besides, in a case where an optical gain is not dependent on the direction of the optical signal, bidirectional optical signals can be amplified simultaneously by one optical amplifier.
Optical amplifiers available now are ones that employ semiconductor lasers, or optical fiber Raman or Brillouin amplifiers which utilize a stimulated scattering phenomenon which occurs in an optical fiber.
However, even if the incident light level is fixed, there arises a problem that a stable output cannot be obtained.